1. Technical Field
The present disclosure relates to oscillators using bulk acoustic wave (BAW) resonators. It is more specifically directed to a method for manufacturing monolithic oscillators based on BAW resonators.
2. Description of the Related Art
Oscillators are mainly used in electronic devices to provide clock signals at reference frequencies. Currently, an oscillator comprises oscillating circuit elements and a quartz resonator which enables accurately setting the oscillation frequency.
An alternative to quartz oscillators is the use of oscillators based on BAW resonators. The use of BAW resonators enables to implement higher oscillation frequencies, for example, approximately ranging from a few hundreds of MHz to a few tens of GHz. Lower clock frequencies may also be generated by using, at the oscillator output, a frequency-dividing circuit. Further, BAW resonators have the advantage of having a low bulk and a good quality factor.
It has also been provided to form a monolithic oscillator using a BAW resonator, that is, an oscillator in which the oscillating circuit elements and the resonator are assembled in and on a same integrated circuit chip. The oscillating circuit elements may be formed in and on a semiconductor substrate, for example, a silicon wafer. The BAW resonator is then deposited above this substrate and connected to the oscillating circuit elements. Such an oscillator has the advantage of being very compact and of providing good electric performances.
FIG. 1 is a cross-section view schematically showing a BAW resonator 1 formed on a semiconductor substrate 3. Resonator 1 comprises a resonator core or piezoelectric resonator 5, formed of two electrodes 5a, 5c between which is arranged a layer 5b of a piezoelectric material. When an electric field is created in the piezoelectric layer 5b by application of a potential difference between the two electrodes 5a, 5c, this results in a mechanical disturbance in the form of acoustic waves.
These waves propagate across the resonator thickness. The fundamental resonance establishes when the acoustic wavelength in the piezoelectric material substantially corresponds to twice the thickness of piezoelectric layer 5b. Schematically, a BAW resonator behaves as an on switch at the resonance frequency and as an off switch at a so-called antiresonance frequency.
An acoustic isolation device is provided between the resonant core and the substrate to avoid losing acoustic waves in the substrate. There mainly exist two types of BAW resonators: BAW resonators deposited on a membrane and BAW resonators mounted on a substrate.
BAW resonators deposited on a membrane, such as resonator 1, are better known as FBARs (Film Bulk Acoustic Wave Resonators) or TFRs (Thin Film Resonators). They comprise an air cavity 7 between the resonator core 5 and the substrate 3.
BAW resonators mounted on the substrate, better known as SMRs (Solidly Mounted Resonators), are generally isolated from the substrate by a Bragg mirror.
FIG. 2 shows a simplified electric diagram of an oscillator with a BAW resonator 25. This oscillator comprises various elements of a circuit 23, connected between a high voltage supply terminal VCC and a terminal of low voltage, for example, the ground, and a BAW resonator 25, connected to the circuit elements 23.
Circuit 23 especially comprises active elements capable of sustaining oscillations and of amplifying output signal OUT, and passive elements, for example, capacitors. BAW resonator 25 enables to select the circuit oscillation frequency.
FIG. 3 shows the circuit of FIG. 2, and shows in more detailed fashion circuit elements 23 in the case of a Colpitts oscillator. In this example, circuit elements 23 include a MOS transistor 31 series-connected with a current source 33, between high supply voltage terminal VCC and the ground. Two capacitors 35 and 37 are series-connected between the gate of transistor 31 and the ground. A resistor 39 is connected between high voltage supply terminal VCC and the gate of transistor 31. The terminal common to capacitors 35 and 37 is connected to the drain of transistor 31. BAW resonator 25 is connected between the gate of transistor 31 and the ground. The oscillator output is connected to the source of transistor 31.
The amplification of the output signal and the sustaining of the oscillations are performed by transistor 31 and current source 33. The frequency of output signal OUT is especially linked to the resonance frequency of resonator 25 and to the values of capacitors 35 and 37.
In practice, it is difficult to obtain an oscillation frequency with the desired accuracy.
A first source of inaccuracy is due to the manufacturing dispersions of BAW resonators. Indeed, methods of deposition of the different layers of a BAW resonator do not enable to obtain a resonance frequency at the desired accuracy. Substantial variations of the resonance frequency can especially be observed between resonators formed on a same substrate wafer.
For this reason, as illustrated in FIG. 1, a frequency adjustment layer 9, for example made of silicon nitride, is provided at the surface of resonator 1. The presence of this layer modifies the behavior of the resonator and especially its resonance and antiresonance frequencies. In a manufacturing step, the thickness of layer 9 is adjusted by local etching, for example, by ion etching, to come closer to the resonance frequency.
Despite this adjustment, the accuracy of BAW resonators is not ideal. As an example, for resonance frequencies on the order of 2 GHz, resonance frequency differences on the order of 2 MHz can currently be observed between BAW resonators formed on a same substrate wafer.
A second source of inaccuracy results from the manufacturing dispersions of the elements of circuit 23. Indeed, despite the attention brought to the forming of these elements, behavior differences between circuits formed in and on a same substrate wafer can be observed. This also currently results in dispersions on the order of 2 MHz on the oscillation frequency of the final circuit.
The two above-mentioned sources of inaccuracy may add up, which results in a lack of accuracy of the oscillation frequency of oscillators using BAW resonators, on the order of 4 MHz in the case of the above examples.
To overcome this lack of accuracy, a variable capacitance, for example formed by a network of switched capacitors, is generally provided in circuit 23. The frequency of the output signal of each oscillator can thus be finely corrected in a final calibration step. Currently, this step occurs when the BAW resonator is assembled and connected to circuit 23 and the oscillator is powered.
A disadvantage of this calibration mode is that, to be able to compensate for the above-mentioned significant inaccuracy of the oscillation frequency, a large network of switched capacitors is provided. This results in a degradation of the electric performances of the oscillator, especially linked to the losses due to the unused switched capacitors of the network.